Serotonin (5-hydroxytryptamine, 5-HT) is a neurotransmitter that modulates central and peripheral functions by acting on neurons, smooth muscle, and other cell types. 5-HT is involved in the control and modulation of multiple physiological and psychological processes. In the central nervous system (CNS), 5-HT regulates mood, appetite, and other behavioral functions. In the GI system, 5-HT plays a general prokinetic role and is an important mediator of sensation (e.g., nausea and satiety) between the GI tract and the brain. Dysregulation of the peripheral 5-HT signaling system has been reported to be involved in the etiology of several conditions (see for example: Mawe, G. M. & Hoffman, J. M. Serotonin Signalling In The Gut-functions, Dysfunctions And Therapeutic Targets. Nature Reviews. Gastroenterology & Hepatology 10, 473-486 (2013); Gershon, M. D. 5-hydroxytryptamine (serotonin) In The Gastrointestinal Tract. Current Opinion in Endocrinology, Diabetes, and Obesity 20, 14-21 (2013); Lesurtel, M., Soll, C., Graf, R. & Clavien, P.-A. Role of Serotonin In The Hepato-gastrointestinal Tract: An Old Molecule For New Perspectives. Cellular And Molecular Life Sciences: CMLS 65, 940-52 (2008)). These include osteoporosis (e.g. Kode, A. et al. FOXO1 Orchestrates The Bone-suppressing Function Of Gut-derived Serotonin. The Journal of Clinical Investigation 122, 3490-503 (2012); Yadav, V. K. et al. Pharmacological Inhibition Of Gut-derived Serotonin Synthesis Is A Potential Bone Anabolic Treatment For Osteoporosis. Nature Medicine 16, 308-12 (2010); Yadav, V. K. et al. Lrp5 Controls Bone Formation By Inhibiting Serotonin Synthesis In The Duodenum. Cell 135, 825-37 (2008)), cancer (e.g. Liang, C. et al. Serotonin Promotes The Proliferation Of Serum-deprived Hepatocellular Carcinoma Cells Via Upregulation Of FOXO3a. Molecular Cancer 12, 14 (2013); Soll, C. et al. Serotonin Promotes Tumor Growth In Human Hepatocellular Cancer. Hepatology 51, 1244-1254 (2010); Pai, V. P et al. Altered Serotonin Physiology In Human Breast Cancers Favors Paradoxical Growth And Cell Survival. Breast Cancer Research: BCR 11, R81 (2009); Engelman, K., Lovenberg, W. & Sjoerdsma, A. Inhibition Of Serotonin Synthesis By Para-chlorophenylalanine In Patients With The Carcinoid Syndrome. The New England Journal of Medicine 277, 1103-8 (1967)), cardiovascular (e.g. Robiolio, P. A. et al. Carcinoid Heart Disease: Correlation of High Serotonin Levels With Valvular Abnormalities Detected by Cardiac Catheterization and Echocardiography. Circulation 92, 790-795 (1995).), diabetes (e.g. Sumara, G., Sumara, O., Kim, J. K. & Karsenty, G. Gut-derived Serotonin Is A Multifunctional Determinant To Fasting Adaptation. Cell Metabolism 16, 588-600 (2012)), atherosclerosis (e.g. Ban, Y. et al. Impact Of Increased Plasma Serotonin Levels And Carotid Atherosclerosis On Vascular Dementia. Atherosclerosis 195, 153-9 (2007)), as well as gastrointestinal (e.g. Manocha, M. & Khan, W. I. Serotonin and GI Disorders: An Update on Clinical and Experimental Studies. Clinical and Translational Gastroenterology 3, e13 (2012); Ghia, J.-E. et al. Serotonin Has A Key Role In Pathogenesis Of Experimental Colitis. Gastroenterology 137, 1649-60 (2009); Sikander, A., Rana, S. V. & Prasad, K. K. Role Of Serotonin In Gastrointestinal Motility And Irritable Bowel Syndrome. Clinica Chimica Acta; International Journal of Clinical Chemistry 403, 47-55 (2009); Spiller, R. Recent Advances In Understanding The Role Of Serotonin In Gastrointestinal Motility In Functional Bowel Disorders: Alterations In 5-HT Signalling And Metabolism In Human Disease. Neurogastroenterology and Motility: The Official Journal of The European Gastrointestinal Motility Society 19 Suppl 2, 25-31 (2007); Costedio, M. M., Hyman, N. & Mawe, G. M. Serotonin And Its Role In Colonic Function And In Gastrointestinal Disorders. Diseases of the Colon and Rectum 50, 376-88 (2007); Gershon, M. D. & Tack, J. The Serotonin Signaling System: From Basic Understanding To Drug Development For Functional GI Disorders. Gastroenterology 132, 397-414 (2007); Mawe, G. M., Coates, M. D. & Moses, P. L. Review Article: Intestinal Serotonin Signalling In Irritable Bowel Syndrome. Alimentary Pharmacology & Therapeutics 23, 1067-76 (2006); Crowell, M. D. Role Of Serotonin In The Pathophysiology Of The Irritable Bowel Syndrome. British Journal of Pharmacology 141, 1285-93 (2004)), pulmonary (e.g. Lau, W. K. W. et al. The Role Of Circulating Serotonin In The Development Of Chronic Obstructive Pulmonary Disease. PloS One 7, e31617 (2012); Egermayer, P., Town, G. I. & Peacock, A. J. Role Of Serotonin In The Pathogenesis Of Acute And Chronic Pulmonary Hypertension. Thorax 54, 161-168 (1999)), inflammatory (e.g. Margolis, K. G. et al. Pharmacological Reduction of Mucosal but Not Neuronal Serotonin Opposes Inflammation In Mouse Intestine. Gut doi: 10.1136/gutjnl-2013-304901 (2013); Duerschmied, D. et al. Platelet Serotonin Promotes The Recruitment Of Neutrophils To Sites Of Acute Inflammation In Mice. Blood 121, 1008-15 (2013); Li, N. et al. Serotonin Activates Dendritic Cell Function In The Context Of Gut Inflammation. The American Journal of Pathology 178, 662-71 (2011)), or liver diseases or disorders (e.g. Ebrahimkhani, M. R. et al. Stimulating Healthy Tissue Regeneration By Targeting The 5-HT2B Receptor In Chronic Liver Disease. Nature Medicine 17, 1668-73 (2011)). The large number of pharmaceutical agents that block or stimulate the various 5-HT receptors is also indicative of the wide range of medical disorders that have been associated with 5-HT dysregulation (see for example: Wacker, D. et al. Structural Features For Functional Selectivity At Serotonin Receptors. Science (New York, N.Y.) 340, 615-9 (2013)).
The rate-limiting step in 5-HT biosynthesis is the hydroxylation of tryptophan by dioxygen, which is catalyzed by tryptophan hydroxylase (TPH; EC 1.14.16.4) in the presence of the cofactor (6R)-L-erythro-5,6,7,8-tetrahydrobiopterin (BH4). The resulting oxidized product, 5-hydroxytryptophan (5-HTT) is subsequently decarboxylated by an aromatic amino acid decarboxylase (AAAD; EC 4.1.1.28) to produce 5-HT. Together with phenylalanine hydroxylase (PheOH) and tyrosine hydroxylase (TH), TPH belongs to the pterin-dependent aromatic amino acid hydroxylase family.
Two vertebrate isoforms of TPH, namely TPH1 and TPH2, have been identified. TPH1 is primarily expressed in the pineal gland and non-neuronal tissues, such as enterochromaffin (EC) cells located in the gastrointestinal (GI) tract. TPH2 (the dominant form in the brain) is expressed exclusively in neuronal cells, such as dorsal raphe or myenteric plexus cells. The peripheral and central systems involved in 5-HT biosynthesis are isolated, with 5-HT being unable to cross the blood-brain barrier. Therefore, the pharmacological effects of 5-HT can be modulated by agents affecting TPH in the periphery, mainly TPH1 in the gut.
A small number of phenylalanine-derived TPH1 inhibitors are known. One example, p-chlorophenylalanine (pCPA), a very weak and unselective irreversible inhibitor of TPH, has proven effective in treating chemotherapy-induced emesis, as well as diarrhea, in carcinoid tumor patients. However, pCPA is distributed centrally and, as a result, its administration has been linked to the onset of depression and other alterations of CNS functions in patients and animals. p-Ethynyl phenylalanine is a more selective and more potent TPH inhibitor than pCPA (Stokes, A. H. et al. p-Ethynylphenylalanine: A Potent Inhibitor Of Tryptophan Hydroxylase. Journal of Neurochemistry 74, 2067-73 (2000), but also affects central 5-HT production and, like pCPA, is believed to irreversibly interfere with the synthesis of TPH (and possibly other proteins).
More recently, bulkier phenylalanine-derived TPH inhibitors have been reported to reduce intestinal 5-HT concentration without affecting brain 5-HT levels (Zhong, H. et al. Molecular dynamics simulation of tryptophan hydroxylase-1: binding modes and free energy analysis to phenylalanine derivative inhibitors. International Journal of Molecular Sciences 14, 9947-62 (2013); Ouyang, L. et al. Combined Structure-Based Pharmacophore and 3D-QSAR Studies on Phenylalanine Series Compounds as TPH1 Inhibitors. International Journal of Molecular Sciences 13, 5348-63 (2012); Camilleri, M. LX-1031, A Tryptophan 5-hydroxylase Inhibitor, And Its Potential In Chronic Diarrhea Associated With Increased Serotonin. Neurogastroenterology and Motility: The Official Journal of The European Gastrointestinal Motility Society 23, 193-200 (2011); Cianchetta, G. et al. Mechanism of Inhibition of Novel Tryptophan Hydroxylase Inhibitors Revealed by Co-crystal Structures and Kinetic Analysis. Current chemical genomics 4, 19-26 (2010); Jin, H. et al. Substituted 3-(4-(1,3,5-triazin-2-yl)-phenyl)-2-aminopropanoic Acids As Novel Tryptophan Hydroxylase Inhibitors. Bioorganic & Medicinal Chemistry Letters 19, 5229-32 (2009); Shi, Z.-C. et al. Modulation Of Peripheral Serotonin Levels By Novel Tryptophan Hydroxylase Inhibitors For The Potential Treatment Of Functional Gastrointestinal Disorders. Journal of medicinal chemistry 51, 3684-7 (2008); Liu, Q. et al. Discovery And Characterization of Novel Tryptophan Hydroxylase Inhibitors That Selectively Inhibit Serotonin Synthesis In The Gastrointestinal Tract. The Journal of Pharmacology and Experimental Therapeutics 325, 47-55 (2008)).
There is a current need to selectively reduce intestinal 5-HT levels as a means for treating and preventing 5-HT-associated diseases. The TPH1 inhibitors described herein are intended to address this need.